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Humans are sexually dimorphic: men are about 15% bigger than women. The usual – and generally accepted – explanation for this size difference is male/male competition for mates. Weston and colleagues measured dry skulls from South Africa to explore which features would deviate from ontogenetic scaling. That is, which features on male skulls would be more than 15% larger than the same features on female skulls?

The white lines indicate those features of the male skull which are larger than expected by ontogenetic scaling. The vertical black line represents the baseline of comparison. There is an additional feature of male skulls that is larger than expected: orbital width (men’s eye sockets are wider).

The white lines indicate those features of the male jaw which are larger than expected by ontogenetic scaling. The vertical black line represents the baseline of comparison. There are two additional features of male jaws that are larger than expected that are not indicated here: ramus (vertical bone at the back of the jaw) height and width at its uppermost.

Because these facial features exhibit greater than 15% variation between males and females, they are likely to result from a separate evolutionary dynamic than caused the bodily dimorphism. Absent any compelling argument from natural selection, sexual selection – female preference – is the most likely candidate explanation. Weston & her colleagues suggest that human females may have preferred males who appeared less aggressive (Weston et.al. 2004).

Weston’s argument offers a solution to a puzzlement that I have had about male facial attractiveness: throughout the majority of human prehistory most men’s facial features would not be visible since we are a long-haired, bearded animal. At the risk of appearing to make a bad pun, the fact that the bulk of these masculine facial characteristics are visible on an unshaven face adds significantly to the face validity of the assertion.